The present invention is directed to improved processing methods for polybutadiene rubber compositions suitable for molded golf ball core constructions. More particularly, the present invention is directed to improved processing methods for polybutadiene rubber compositions having a high Mooney viscosity and/or a high molecular weight and a low dispersity. The use of such polybutadiene rubber increases the resiliency of a golf ball without increasing the hardness of the ball.
Two of the principal properties involved in the performance of golf balls are resilience and hardness. Resilience is determined by the coefficient of restitution (referred to as “C.O.R.”), also expressed as the constant “e”, which is the ratio of the relative velocity of two elastic spheres after direct impact to that before impact. More generally, the constant “e” is the ratio of the outgoing velocity to the incoming velocity of a rebounding ball. The coefficient of restitution (i.e. “e”) can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision.
Hardness is determined as the deformation (i.e. compression) of the ball under various load conditions applied across the ball's diameter. The lower the compression value, the harder the material.
Resilience (C.O.R.), along with additional factors such as clubhead speed, angle of trajectory, and ball configuration (i.e. dimple pattern), generally determine the distance a ball will travel when hit. Clubhead speed and the angle of trajectory are factors not easily controllable, particularly by golf ball manufacturers. Thus, the factors of concern to manufacturers are the coefficient of restitution (C.O.R.) and the surface configuration of the ball.
The coefficient of restitution of a golf ball is generally measured by propelling a ball at a given speed against a hard surface and electronically measuring the ball's incoming and outgoing velocity. The coefficient of restitution must be carefully controlled in all commercial golf balls in order for the ball to be within the specifications regulated by the United States Golfers Association (“U.S.G.A.”).
The U.S.G.A. standards dictate that a “regulation” ball can not have an initial velocity (i.e. the speed off the club) exceeding 255 feet per second (250 feet per second with a 2% tolerance). Since the coefficient of restitution of a ball is related to the ball's initial velocity (i.e. as the C.O.R. of a ball is increased, the ball's initial velocity will also increase), it is highly desirable to produce a ball having a sufficiently high coefficient of restitution to closely approach the U.S.G.A. limit on initial velocity, while having an ample degree of hardness (i.e. impact resistance) to produce enhanced durability.
The coefficient of restitution (C.O.R.) for a solid core golf ball is a function of the composition of the molded core and of the cover. Copending U.S. patent application Ser. No. 09/226,340, filed Jan. 6, 1999 and titled “Golf Ball Cores Formed from Ultra-high Mooney Viscosity Butadiene Rubber”, is assigned to the same assignee as the present invention and is hereby incorporated by reference. This copending application discloses an improved polybutadiene composition that exhibits a high Mooney viscosity and/or a high molecular weight and low dispersity that, when utilized to formulate golf ball cores, produces golf balls exhibiting enhanced C.O.R. without increasing hardness.
However, while high molecular weight polybutadiene rubbers exhibit improved C.O.R., they are very difficult to mill. That is, they tend to “bag”, crumble, and break into pieces during the milling process. Bagging occurs when the core stock becomes loose and “bags” or droops down away from a roller associated with a conventional two-roll mill (described further below) thus preventing the core stock from being cut by knife blades associated with the mill.
Conventional processing aids can be added to the butadiene rubber (BR) core stock to improve the core stock's milling characteristics. However, the processing aids disadvantageously lower the core stock's coefficient of restitution. Accordingly, it is an object of the present invention to provide improved processing methods for polybutadiene rubber compositions that exhibit a high Mooney viscosity and/or a high molecular weight and a low dispersity.
These and other objects and features of the invention will be apparent from the following summary and description of the invention and from the claims.